Conventionally, a development system is known, which supplies a new development agent to a development device and discharges an old development agent from the development device to maintain a performance of the development agent in the development device.
In such development system, a technique is known, which mixes a small amount of carrier in a toner cartridge containing a toner supplied to the development device and supplies it together with the toner in order to replace the carrier gradually (for example, see Japanese Patent Application Laid-open No. H09-185177).
However, in the conventional development system, the carrier is supplied to the development device and discharged out of it together with the supply of the toner. Thus, the carrier in the development device is seldom supplied if an output image has a low print ratio, and as a result, the carrier in the development device is not sufficiently replaced. As such, it has a problem that, if the carrier in the development device is not sufficiently replaced, the development agent is deteriorated to generate a poor image quality.